1. Field of the Invention
The present invention is directed to a method for the operation of a magnetic resonance apparatus, of the type wherein a positional change of a region of an examination subject to be imaged relative to an imaging volume of the apparatus is acquired with orbital navigator echos.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for acquiring images of the inside of the body of a subject to be examined. In a magnetic resonance apparatus, rapidly switched gradient fields are superimposed on a static basic magnetic field. For triggering magnetic resonance signals, radio-frequency signals are emitted into the examination subject, the magnetic resonance signals that are triggered are being detected, and image data sets and magnetic resonance images being produced on the basis thereof. The magnetic resonance signals are detected by a radio-frequency system, are demodulated in phase-sensitive fashion and converted into complex quantities by sampling and analog-to-digital conversion. These complex quantities are stored as data points in a k-space dataset from which an appertaining image dataset, and thus a magnetic resonance image, can be reconstructed with a multi-dimensional Fourier transformation.
Functional imaging in medicine refers to all methods that utilize a repeated scanning of a structure of organs and tissues in order to image temporally changing processes such as physiological functions or pathological events. In the narrower sense, in magnetic resonance technology functional imaging refers to measuring methods that make it possible to identify and image sensory stimuli and/or areolae in the nervous system stimulated by a motor, sensory or cognitive task, particularly the cerebral areolae of a patient.
The BOLD effect (Blood Oxygen Level Dependent) is the basis of functional magnetic resonance imaging. The BOLD effect is based on different magnetic properties of oxygenated and de-oxygenated hemoglobin in the blood. An intensified neural activity in the brain is assumed to be locally connected with an increased delivery of oxygenated blood, which causes a corresponding intensity boost at a corresponding location in a magnetic resonance image generated with a gradient echo sequence.
In functional magnetic resonance imaging, for example, three-dimensional image datasets of the brain are registered every two through four seconds, for example with an echo planar method. After many image datasets have been registered at various points in time, the image datasets can be subtracted from one another, for example for forming images referred to as activation images, i.e. they can be compared to one another in view of signal differences for the identification of active brain areas. Even the slightest positional change of the brain during the overall exposure time span of the functional magnetic resonance imaging leads to undesirable signal differences that mask the brain activation that is being sought.
In one embodiment of a functional magnetic resonance imaging, image datasets of a region to be imaged are generated with an identical location coding in a time sequence. A retrospective motion correction of the image datasets is implemented following thereupon. Differences between the image datasets that are a result of a positional change of the imaged region with respect to the apparatus during the time sequence thus can be identified and corrected. To that end, a global difference between two image datasets is minimized, with a positional change between the two image datasets, that can be described by motion parameters, being linearized by a Taylor development of the first order, with the assumption of a uniform body motion. The minimization ensues iteratively by the motion parameters being repeatedly estimated with the linearization and applied to one of the two image datasets with interpolations. Such methods are known as Gauss-Newton method in the literature. For a more detailed description, the book by R. S. J. Frackowiak et al., Human Brain Function, Academic press, 1996, particularly Chapter 3, pages 43 through 58 is referenced as an example.
In another embodiment of a functional magnetic resonance imaging, a prospective motion correction is implemented during the execution of the functional magnetic resonance imaging. To that end, possible positional changes, i.e. rotations and translation of the region to be imaged, are acquired from image dataset to image dataset by, for example, orbital navigator echos and a location coding is correspondingly adapted during the execution.
An orbital navigator echo is a magnetic resonance signal that is characterized by a circular k-space path and that is generated by a specific navigator sequence. A navigator echo is registered just like a magnetic resonance signal employed for image generation and is correspondingly stored in a navigator echo dataset as complex quantities for data points of k-space that form the circular k-space path. A positional change between the points in time can be determined on the basis of orbital navigator echos that are generated at different points in time. To that end, for example, the navigator sequence is implemented before each generation of an image dataset, a navigator echo is registered, and an appertaining navigator echo dataset is compared to a reference navigator echo dataset for acquiring positional changes.
As is known, a relationship between the image space and k-space exists via a multi-dimensional Fourier transformation. According to the shift theorem of the Fourier transformation, a translation of the region to be imaged in the image space is expressed as a modified phase of complex quantities of data points of k-space. A rotation of the imaged region in the image space effects the same rotation of appertaining data points in k-space. In order to decouple (distinguish) a rotation from a translation in k-space, only amounts of the complex quantities are considered for rotations. A rotation of the imaged region relative to a reference point in time thus can be identified by a comparison of amount values of the navigator echo dataset to those of the reference navigator echo dataset. The phase values are compared for a translation.
For acquiring arbitrary positional changes in three-dimensional space, a respective orbital navigator echo is generated in three planes that are orthogonal to one another. Given positional changes with rotations up to xc2x18xc2x0 and translation up to xc2x18 mm, an imprecision of up to approximately xc2x11.5xc2x0 and xc2x11.5 mm is to be expected for an arbitrary positional change in this contest. Such imprecision can be improved by a repetition of the orbital navigator echos for specifically directed positional changes. After a comparison of a first navigator echo dataset to the reference navigator echo dataset corresponding to an identified positional change, a location coding is adapted for this purpose, and a second navigator echo dataset is registered with the adapted location coding. This dataset in turn is compared to the reference navigator echo dataset, which again leads to the adaptation of the location coding when a positional change is found. Particularly given positional change with a rotational component, however, no improved precision can be achieved in view of the rotational component. The above-discussed use of the orbital navigator echos is explained in greater detail, for example, in the article by H. A. Ward et al., xe2x80x9cProspective Multiaxial Motion Correction for fMRIxe2x80x9d, Magnetic Resonance in Medicine 43 (2000), pages 459 through 469.
An object of the invention is to provide an improved method of the type initially described which allows positional changes to be acquired with, among other things, a high precision.
This object is achieved in accordance with the invention in a method for the operation of a magnetic resonance apparatus, which allows a positional change of a region of an examination subject to be imaged relative to an imaging volume of the apparatus to be acquired with orbital navigator echos, wherein at least one reference dataset with data points is generated, with the data points in k-space at least on a partial surface of a spherical surface, being arranged to occupy the partial surface in surface-covering fashion, and wherein the partial surface extends at least around a great circle of the spherical surface corresponding to a maximally expected angular range of the positional change; and wherein for acquiring the positional change, a dataset of at least one of the navigator echos is compared to the reference dataset.
Compared to the above-described method known, wherein only data points are arranged on the spherical surface on three circular paths that are orthogonal relative to one another, the surface-covering occupation of the partial surface with data points achieves clearly improved precision in the acquisition of positional changes. This is particularly true of a portion of a positional change that is to be attributed to a rotation of the region to be imaged.
In an embodiment, the partial surface is equal to the spherical surface. As a result, nearly arbitrary rotations in the broad range from greater than 0xc2x0 through less than 180xc2x0 can be acquired with high precision.
In another embodiment, a dataset of a combination of two orbital navigator echos, whose circular k-space paths define two planes that are orthogonal relative to one another, is compared to the reference dataset for acquiring the positional change. Compared to the known method, wherein a combination of three orbital navigator echos that are orthogonal to one another is required, an arbitrary three-dimensional positional change thus can already be acquired with high precision with the combination of only two orbital navigator echos. In applications wherein successive combinations of the aforementioned type are multiply generated for acquiring arbitrary three-dimensional positional changes, a shortening of a pickup duration is achieved compared to the known method. Although the pick-up duration for the reference dataset is lengthened compared to the known method, this is more than compensated by the shortened pickup duration for the combinations of orbital navigator echos with a specific number of combinations.